Micro-Electro-Mechanical Systems (or MEMS) devices are becoming increasingly prevalent as low-cost, compact devices having a wide range of applications. Uses include pressure sensors, accelerometers, gyroscopes, microphones, digital mirror displays, microfluidic devices, biosensors, chemical sensors, and others.
MEMS transducers include both actuators and sensors. In other words they typically convert an electrical signal into a motion, or they convert a motion into an electrical signal. They are typically made using standard thin film and semiconductor processing methods. As new designs, methods and materials are developed, the range of usages and capabilities of MEMS devices can be extended.
MEMS transducers are typically characterized as being anchored to a substrate and extending over a cavity in the substrate. Three general types of such transducers include a) a cantilevered beam having a first end anchored and a second end cantilevered over the cavity; b) a doubly anchored beam having both ends anchored to the substrate on opposite sides of the cavity; and c) a clamped sheet that is anchored around the periphery of the cavity. Type c) is more commonly called a clamped membrane, but the word membrane will be used in a different sense herein, so the term clamped sheet is used to avoid confusion.
Sensors and actuators can be used to sense or provide a displacement or a vibration. For example, the amount of deflection δ of the end of a cantilever in response to a stress σ is given by Stoney's formulaδ=3σ(1−ν)L2/Et2  (1),where ν is Poisson's ratio, E is Young's modulus, L is the beam length, and t is the thickness of the cantilevered beam. In order to increase the amount of deflection for a cantilevered beam, one can use a longer beam length, a smaller thickness, a higher stress, a lower Poisson's ratio, or a lower Young's modulus. The resonant frequency of vibration of an undamped cantilevered beam is given byf=ω0/2π=(k/m)1/2/2π  (2),where k is the spring constant and m is the mass. For a cantilevered beam of constant width w, the spring constant k is given byk=Ewt3/4L3  (3).It can be shown that the dynamic mass m of an oscillating cantilevered beam is approximately one quarter of the actual mass of ρwtL (ρ being the density of the beam material), so that within a few percent, the resonant frequency of vibration of an undamped cantilevered beam is approximatelyf˜(t/2πL2)(E/ρ)1/2  (4).For a lower resonant frequency one can use a smaller Young's modulus, a smaller thickness, a longer length, or a larger density. A doubly anchored beam typically has a lower amount of deflection and a higher resonant frequency than a cantilevered beam having comparable geometry and materials. A clamped sheet typically has an even lower amount of deflection and an even higher resonant frequency.
Based on material properties and geometries commonly used for MEMS transducers the amount of deflection can be limited, as can the frequency range, so that some types of desired usages are either not available or do not operate with a preferred degree of energy efficiency, spatial compactness, or reliability. In addition, typical MEMS transducers operate independently. For some applications, independent operation of MEMS transducers is not able to provide the range of performance desired.
Ultrasonic imaging is an important analytical tool and, as such, is useful in several applications, including, for example, medical diagnostics and nondestructive testing. It is a noninvasive and benign way of imaging features that are below the surface of an object and therefore not readily observable by optical methods. Ultrasonic waves are sent into an object, echo signals are received, and an image is obtained by analyzing and interpreting the echo signals. For medical ultrasonography the frequency of the ultrasonic pulses typically ranges from around 1 MHz to around 20 MHz, with the lower frequencies in this range being used to image organs or other structures deep within the body, and the higher frequencies being used to image structures (in greater detail) that are closer to the surface.
Ultrasonic waves are typically produced by a piezoelectric transducer array housed in a probe. Electrical pulses from a pulse source cause the piezoelectric transducers in the array to oscillate. By controlling the timing of pulsing of various transducers in the array, an arc-shaped wave front can be provided in a process that is sometimes called beam forming. The ultrasonic wave travels into the object and comes into focus at a desired depth. Impedance matching materials on the face of the piezoelectric transducer array probe enable the waves to be transmitted efficiently into the object.
For medical ultrasonography a water-based gel is typically placed between the patient's skin and the probe to promote efficient transmission of waves and reception of echoes. The ultrasonic wave is partially reflected from the layers between different tissues. In particular, the ultrasonic wave is reflected anywhere there are density changes. The reflected ultrasonic echo induces one or more transducers in the array to vibrate. There is typically significant attenuation of the ultrasonic wave as it passes through the object and is reflected, so that the reflected wave (the echo) has a much lower amplitude than the ultrasonic wave that was sent into the object. The transducers convert the reflected waves into electrical signals that are amplified and sent to a controller for processing and transforming into a digital image.
In a conventional ultrasonic transducer array, the same transducers are used for both transmitting waves into the object and receiving reflected waves. A block diagram of a portion of a conventional ultrasonic imaging system 10 is shown in FIG. 1. A controller 30 includes circuitry for controlling electrical pulses to be sent to the transducers of ultrasonic transducer array 20. Pulsing circuitry 40 includes a high voltage power supply, a pulse generator, and a high voltage amplifier for providing electrical pulses having an amplitude typically between 20V and 200V. The high voltage pulses are selectively passed through a transmit/receive switch 25 to ultrasonic transducer array 20. Reflected waves are converted to echo electrical signals by ultrasonic transducer array 20 and are selectively passed through transmit/receive switch 25 to receiver circuitry 45. Receiver circuitry 45 includes a low noise amplifier to amplify the echo electrical signals. The amplified echo signals are sent to signal processing circuitry in the controller 30 for processing the echo signals and transforming them into a digital image. The image can be viewed on display 36, saved in storage 38, or printed on a printing system (not shown).
The purpose of the transmit/receive switch 25 is primarily to isolate receiver circuitry 45 from pulse circuitry 40. Since the pulse circuitry 40 is otherwise connected to receiver circuitry 45 at transducer array 20, high voltage signals from pulse circuitry 40 may damage the low noise amplifier or other sensitive components of the receiver circuitry.
Accordingly, there is a need for a MEMS transducer design and method of operation that enables low cost and spatially compact transducer arrays for ultrasonic transmitters and receivers, such that the transmitter portion of the transducer is electrically isolated from the receiver portion, thereby reducing or even eliminating the need for a transmit/receive switch.